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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110 Fax: 020 7405 4967
 

Rainwater Harvesting

Water ButtRainwater harvesting involves the use of captured rainwater, usually from a roof catchment, which otherwise would have soaked into the ground, evaporated or entered the drainage system. Once captured, the water can be drawn on for a variety of uses from irrigating crops or gardens, as toilet flush water, in water features and occasionally as a source of drinking water. Watering a garden with rainwater collected in a water butt is a rudimentary form of rainwater harvesting.

Where there is negligible potential human contact the rainwater will usually only require coarse filtration to prevent leaf litter, debris and small animals entering the system. If the rainwater is to provide a potable water supply, thorough treatment is required which makes this use uncommon.

During rainfall events the first 'flush' of water usually has the lowest water quality due to contamination from leaf litter, bird droppings and wind-blown pollutants that have adhered to the roof surface or guttering. For this reason, many rainwater harvesting systems divert the 'first flush' of water so that it is not used.

The amount of rainwater that can be harvested is a function of rainfall received and plan roof area. For example in Northern Ireland where 2004 Annual rainfall was just over 1000mm/year, a home with a 100m2 plan roof area could have 'harvested' 60m3 of rainwater, assuming that 60% of rain that falls on a roof catchment is collected and used.

As well as capturing rainwater from roof catchments, rainwater harvesting systems are also being designed in conjunction with SuDS (Sustainable Drainage Systems) to capture the rainwater which infiltrates through permeable paving surfaces. The only addition needed to the rainwater harvesting system is an oil trap to remove oils from the paving surface (e.g. from cars) which are washed down into the system.

In the UK there are corporation tax incentives available to help with the cost of rainwater system investments.

In Germany, a study into rainwater harvesting by the consultants Mall GmbH has shown that 35% of new buildings built in Germany in 2005 were equipped with a rainwater collection system. Turnover from the industry is worth 340 million euros and 5000 jobs have been created.

 

Case Studies

Read about  rainwater harvesting at the Eden Project in Cornwall

 

Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110 Fax: 020 7405 4967

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